This claims the benefit of copending German Applications Nos. 19930720.2, filed Jul. 2, 1999 and 19956692.5, filed Nov. 25, 1999.
The invention relates to a litho strip, comprised of a rolled aluminum alloy, for electrochemical roughening and a method for its manufacture
Very high requirements are placed on the purity and uniformness of litho strip surfaces, and, therefore, special measures must be taken already during casting of the blank material so t hat no oxides or other contaminants can contaminate the metal. Rectangular cast ingots provide the blank material which, after milling off the casting skin, are rolled by means of hot or cold reduction to thin strips. The finish-rolling is performed usually with fine-ground steel rolls so that, as a standard, a so-called mill-finish surface is obtained. The semi-finished product is referred to as offset strip or litho strip and is conventionally rolled to a coil.
As a standard material high-grade aluminium (AA1050) as well as alloys of the type AlMn1 (AA3003, AA3103) are used.
The rolled strip is then further processed to printing plate supports by roughening the strip surface. Mechanical, chemical, and electrochemical roughening processes as well as combinations thereof are known. It is standard practice to perform the electrochemical (EC) roughening in baths based on HCl or HNO3; the topography produced thereby is characterized by fine, round pits of a diameter of  less than 20 xcexcm; the printing plate is rough across the entire surface and exhibits a structure-less appearance (no streakiness effects). The roughened structure is protected by anodization, i.e., by a thin hard oxide layer. By applying a photosensitive photo layer, the printing plate support is transformed into an offset printing plate. The printing plates are irradiated and developed. In the case of positive plates the photosensitive layer is burned in at temperatures in the range of 220-300xc2x0 C. and burning times of 3-10 min.; by this thermal treatment the image points become abrasion-proof so that the printing plate is suitable for high printing editions. In this context, the Al printing plate support should lose as little as possible of its strength because soft plates can no longer be handled without buckling.
The finished printing plate is inserted into the printing machine. Important is an exact clamping of the plate on the printing cylinder so that no movement play will result during the printing process. When the printing plate is not perfectly secured and is thus cyclically subjected to bending or torsional loads during printing, plate cracking occurs according to practical experience in the fast running rotary offset printing machines. The reason for this is fatigue fracture, and the result is an immediate interruption of the printing process. Al-materials for offset printing plates therefore must exhibit a sufficiently high fatigue strength or reversed bending fatigue strength so that plate cracking can be prevented.
It is known that the employed material type has an effect on the reversed bending fatigue strength: as is known from practical experience, offset printing plates made of AlMn alloys (AA3003, AA3103) have a reduced tendency to exhibit plate cracking in comparison to offset printing plates made of high-grade aluminium (AA1050). A disadvantage of the AlMn alloys is an insufficient roughening behaviour in the EC processes. Consequently, the material AA 1050 is preferably employed for EC-roughened plates.
The printing plate manufacturers also know that there is a great difference with respect to the sensitivity toward plate cracking depending on the direction in which the printing plate is taken from a rolled Al strip: when taking the plate parallel to the rolling direction and clamping it such that the previous rolling direction is oriented in the running direction of the printing machine, the plates will crack substantially less frequently, as is known from practical experience, than when taking the plates perpendicularly to the rolling direction (xe2x80x9ctransverse directionxe2x80x9d). For preventing plate cracking, printing plates for rotary offset printing machines are therefore preferably taken in the parallel rolling direction (in xe2x80x9clongitudinal directionxe2x80x9d) from the rolled Al strip. This measure presents a strong limitation with respect to economical considerations for cutting the coils into different printing plate formats.
For increasing the productivity, printing machines have been developed in recent years which require offset printing plates that are oversized with respect to their width and have a width of  greater than 1700 mm. The plates for this new printing machine generation must be taken from the aluminum coil in a direction transverse to the rolling direction because at the moment neither the semi-finished product manufacturers nor the printing plate manufacturers can produce widths of  greater than 1,700 mm. For the new printing machines, which require oversized plates, an Al material is desirable which has a high reversed bending fatigue strength transverse to the rolling direction.
It can be taken from the description that a new requirement profile for the offset printing plates is present on the printing market. The aluminum support used as a substrate should therefore have the following combination of properties:
a high thermal stability so that the Al substrate will not become soft (not recrystallize) during burning of the photosensitive layer;
a good roughening behavior in the EC processes based on HCl and HNO3 so that the Al material can be employed universally;
a high reversed bending fatigue strength, in particular, in the critical transverse direction (relative to the rolling direction) so that the printing plates can be taken from the rolled aluminum coil in any desired orientation.
It is an object of the present invention to develop a litho strip that has a high thermal stability, can be roughened in EC processes based on HCl and HNO3 as efficiently as high-grade aluminium, exhibits after EC roughening a uniform structure-free (streak-free) appearance, and has a high reversed bending fatigue strength perpendicularly to the rolling direction. Moreover, a method is to be provided with which the afore described properties can be produced in the litho strips.
This object is solved according to the invention by the features disclosed in the claims. It was found that technically useful offset printing plates of a width of more than 1,700 mm can be produced with these measures which strips have substantially the same high reversed bending fatigue strength as AA 1050 in the longitudinal direction.
The new litho strip is characterized by a narrowly limited alloy range and by a controlled semi-finished product manufacture with which a fine-grain, recrystallized hot rolled strip can be produced. Also, the further processing must be carried out under controlled conditions so that the microstructure adjusted during the rolling process is maintained.
The development of the new material was carried out in terms of significantly improving the reversed bending fatigue strength of rolled litho strip material in the transverse direction relative to the standard material AA 1050. By means of experiments, it was determined that for this purpose alloy elements are suitable which are in solid solution and/or can be maintained in solid solution in the aluminum mixed crystal: they increase the strength only to a limited extent but have a positive effect on the fatigue behaviour. In this context, especially the elements Mg, Cu, and Fe are of interest.